Effect of Electric Field on Gas Hydrate Nucleation ... - ACS Publications

Article Cite This: Environ. Sci. Technol. XXXX, XXX, XXX−XXX

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Effect of Electric Field on Gas Hydrate Nucleation Kinetics: Evidence for the Enhanced Kinetics of Hydrate Nucleation by Negatively Charged Clay Surfaces Taehyung Park and Tae-Hyuk Kwon* Department of Civil and Environmental Engineering, Korea Advanced Institute of Science and Technology (KAIST), Daejeon, 34141, Korea S Supporting Information *

ABSTRACT: Natural gas hydrates are found widely in oceanic clay-rich sediments, where clay−water interactions have a profound effect on the formation behavior of gas hydrates. However, it remains unclear why and how natural gas hydrates are formed in clay-rich sediments in spite of factors that limit gas hydrate formation, such as small pore size and high salinity. Herein, we show that polarized water molecules on clay surfaces clearly promote gas hydrate nucleation kinetics. When water molecules were polarized with an electric field of 104 V/m, gas hydrate nucleation occurred significantly faster with an induction time reduced by 5.8 times. Further, the presence of strongly polarized water layers at the water−gas interface hindered gas uptake and thus hydrate formation, when the electric field was applied prior to gas dissolution. Our findings expand our understanding of the formation habits of naturally occurring gas hydrates in clay-rich sedimentary deposits and provide insights into gas production from natural hydrate deposits. mineral surfaces.18,19,27 Adsorbed cations depress the activity of the associated water molecules, inhibiting the formation of gas hydrates and retarding nucleation kinetics and phase equilibriums.24,30 Moreover, the capillarity between solid hydrates and liquid water, caused by the small pores in clay minerals, lowers the water activity and inhibits hydrate formation.26−28,31,32 However, it remains unclear and controversial as to whether the presence of polarized water molecules on negatively charged clay surfaces promotes or inhibits gas hydrate formation. One hypothesis is that the strongly polarized water molecules on clay surfaces hardly form clathrate hydrate due to their sufficiently lowered water activity.27 A conflicting hypothesis proposes that the clay surfaces provide nucleation sites for gas hydrates, promoting nucleation kinetics and equilibrium.30,33 These conflicting hypotheses exist due to the experimental difficulty in separating the sole contribution by the negatively charged surfaces from the effect of adsorbed cations. This is because clay particles naturally contain cations to balance their surface charges and inevitably release cations when mixed with water. Therefore, in this work, we investigated the contribution of negatively charged clay surfaces to gas hydrate nucleation by polarizing the water dipoles with an external electric field. A series of controlled hydrate formation tests was conducted, where we monitored the nucleation kinetics of gas hydrate

1. INTRODUCTION Gas hydrates are ice-like crystalline structures composed of hydrogen-bonded water molecules encapsulating gas molecules.1,2 Natural gas hydrates, primarily methane (CH4) hydrates, are considered as prospective energy resources owing to their vast quantities3−5 as well as potential triggers and/or accelerators for geological hazards6−9 and climate change.10−12 Carbon dioxide (CO2) hydrate has been the subject of substantial attention due to the possibility of geologic carbon storage using CO2 hydrate.13,14 Moreover, CH4−CO2 replacement in the hydrate phase has been suggested for the recovery of CH4 and the simultaneous storage of CO2 in natural methane hydrate deposits.15,16 Because of similarities in the crystal structures and physicochemical characteristics between CO2 hydrate and CH4 hydrate, such as the guest molecule size (4.36 and 5.12 Å, respectively) and the clathrate structure type (both are structure I hydrates),17 many laboratory studies have found CO2 hydrate to be a suitable analogue for CH4 hydrate. Substantial amounts of global gas hydrates are accumulated in oceanic clay-rich fine-grained sediments.3,18−21 Because of the isomorphous substitution of cations in clay mineralogy, the surfaces of natural clay minerals are negatively charged and thus unavoidably generate physicochemical interactions between clay and water.22,23 Such clay−water interactions have a critical role in the occurrence of natural gas hydrates in clay-rich sedimentary formations in a number of ways: by causing the depression of water activity by the adsorbed cations,24,25 confinement by the capillarity of small pore sizes,26−29 and the partial ordering of water molecules by negatively charged clay © XXXX American Chemical Society

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October 26, 2017 January 30, 2018 February 2, 2018 February 3, 2018 DOI: 10.1021/acs.est.7b05477 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

Article

Environmental Science & Technology

Figure 1. (a) Setup for the hydrate formation experiment. (b) Schematic drawing of water subjected to an electric field. (c) Spatial distribution of polarization density in the water subjected to the electric potential of 2400 Vdc. (d) Typical temperature trace during the CO2 hydrate formation experiment. The figure depicts when the external electric field was applied for different cases and the estimation of the induction time of gas hydrate nucleation.

were 4 mm apart (Figure 1b). An electric field was applied by imposing a dc voltage of 2400 V to the electrodes using a high voltage power supplier (KSC Korea Switching, Seoul, South Korea). The minimum electric field strength to induce an electrofreezing effect, where the external electric field reduces the supercooling of water, has been reported to range from approximately 105 to 106 V/m.35 Therefore, a dc voltage of 2400 V was chosen to generate a similar electric field strength. The water−quartz setup between the parallel aluminum plates was placed in a high-pressure reaction cell with a tempered glass window. The high-pressure reaction cell was made of stainless steel and had an internal diameter of 50 mm, a height of 180 mm, and a volume of 150 cm3. The temperature of the reaction cell was controlled using a temperature-controlled bath. A thermistor (QTI Sensing Solutions, Boise, ID) submerged in the water droplet on the quartz substrate was used to monitor the temperature and detect hydrate nucleation from exothermic temperature jumps. A pressure transducer (PX302; Omega Engineering, Inc., Norwalk, CT) was housed in the reaction cell to measure the inside gas pressure. The temperature and pressure data over the course of the experiments were acquired using a data acquisition unit (34970A; Keysight Technologies, Santa Rosa, CA). 2.2. Experimental Procedures. The experimental procedure for hydrate formation was as follows. (Step A) The quartz substrate containing 300 μL of fresh DIW was placed between two parallel electrodes in the reaction cell, and the assembled cell was submerged in a temperature-controlled bath at a temperature of 285.15 K. (Step B) The cell was purged with CO2 gas to remove the residual air inside the cell. After purging, the reaction cell was pressurized to 3 MPa with CO2 gas and the pressure was kept constant for 5 h, allowing CO2 gas

when water was subjected to the external electric field. In addition, the effects of electric field applications at different thermodynamic states, such as inside and outside the hydrate stability conditions or before and after the dissolution of hydrate-forming gas, were examined. As a fundamental mechanism of gas hydrate nucleation, whether CO2 hydrate or CH4 hydrate, it is consistent and applicable to any type of gas-forming sI hydrate, and our experimental observations based on CO2 hydrate can be further extended to the electric field strength in double diffuse layers (DDLs) on clay minerals and their impact on natural gas hydrate formation in clay-rich sediments.

2. MATERIALS AND METHODS 2.1. Materials and Test Setup. The experiment was designed to measure the nucleation time of CO2 hydrate in the presence of an electrical field (Figure 1). As hydrate formers, 300 μL of deionized water (DIW) (>107 Ω cm) and commercial 99.9% grade compressed CO2 gas (Sam-O Gas Co., Daejeon, South Korea) were used in this study. Owing to the similar physical characteristics between CO2 hydrate and CH4 hydrate, CO2 was chosen as a suitable analogue for CH4 hydrate and used as a hydrate former. First, 300 μL of DIW was placed on a quartz substrate with an inner diameter of 18 mm and wall height of 1.5 mm, resulting in a disk-shaped DIW droplet with a diameter of 18 mm and thickness of 1.18 mm. Quartz was used as an electrically neutral substrate because of its high purity and well-defined chemical structure. Although quartz may have a weak surface charge (∼−0.32 mC/m2),34 its effect was presumed to be negligible because the applied external electric field was much greater. The substrate was placed between two parallel plates of aluminum electrodes that B

DOI: 10.1021/acs.est.7b05477 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

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Figure 2. Test results of the CO2 hydrate nucleation induction times: (a) temperature traces and exothermic temperature peaks recorded for Case 1, (b) log-normal distributions of CO2 hydrate nucleation induction times for Case 1, (c) temperature traces and exothermic temperature peaks recorded for Case 2, and (d) log-normal distributions of CO2 hydrate induction times for Case 2. The black diamond indicates the time when the external electric field was applied. Case REF refers to the reference test results where the hydrate formation experiments were conducted in the absence of electric field.

(REF) was carried out without the electric field. In Case 1, the electric field was applied prior to step C (i.e., after CO2 dissolution and outside the hydrate stability condition). In Case 2, the electric field was imposed after step C (i.e., after CO2 dissolution and within the hydrate stability condition). In Case 3, the electric field was applied prior to step B (i.e., before CO2 dissolution and outside the hydrate stability condition). Each experimental case was repeated 15 times under the same conditions to obtain the probabilistic distribution of the CO2 hydrate nucleation time. 2.3. Estimation of Electric Fields Using a Finite Element Code. The spatial distribution of the electric field between the electrodes was computed using a finite element code (COMSOL Multiphysics, COMSOL Inc.). The axisymmetric configuration with the water-quartz-electrode geometry was modeled so as to be identical to the test setup, and the magnitude of the applied electric field and the polarization density within water were calculated (Figure S1). As the input parameter, a potential of 2400 Vdc was applied to the two aluminum electrodes to create the electric field. The relative permittivities (or dielectric constants) of water, quartz,

dissolution to water at 285.15 K and 3 MPa. (Step C) Under isochoric conditions, the temperature of the cell was lowered to 274.15 K to bring the pressure−temperature (P−T) condition into the hydrate stability region. (Step D) The P−T conditions for hydrate formation were determined to keep CO2 in the gaseous phase and to ensure the compatibility of the obtained test results with CH4 hydrate as methane in the gaseous phase in natural geologic settings. Nucleation kinetics of gas hydrate can be quantified by the induction time for hydrate nucleation, which is defined as the time required to commence hydrate nucleation under a thermodynamically stable P−T condition.2 The exothermic reaction of hydrate nucleation was detected by a sudden temperature jump. Therefore, the induction time of hydrate nucleation was estimated from the moment when the P−T condition of the system was brought within the hydrate stability condition to the moment of an exothermic temperature jump (Figure 1d). To investigate the effect of electrical fields on gas hydrate nucleation kinetics, an electric field was imposed by applying a potential difference of 2400 Vdc between the two parallel electrodes. Four experimental cases with and without the electric field were tested (Figure 1d). A reference case C


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Figure 3. (a) Enhancement of CO2 hydrate nucleation kinetics caused by the presence of partially aligned water dipoles. When the water is saturated with dissolved CO2, the external electric field polarizes water dipoles and promotes hydrate nucleation, reducing the induction time (Cases 1 and 2). (b) Inhibition of CO2 uptake in the presence of strongly polarized water layers at the water−gas interface. When the external electric field is applied to water prior to gas dissolution, the presence of strongly polarized water layers at the water−gas interface impede gas uptake thereby inhibiting gas hydrate nucleation (Case 3). (c) Variation in electric field with the distance from clay surface at different electrolyte concentrations. The gray area represents the electric field strength deployed in this study. c indicates the molar concentration (mol/L) of 1:1 type electrolyte (NaCl), and 1/κ represents the thickness of the double diffuse layer. (d) Formation of gas hydrate near negatively charged clay surfaces. The water molecules near the mineral surface are strongly bounded and structured, retarding gas diffusion. Meanwhile, the weakly polarized water molecules in the further region are more prone to hydrate formation than the free water. Note that the figure is not drawn to scale.

although the degree of polarization was less than that observed in previous studies, because we applied a smaller electric field.

aluminum, acrylic plastic, and CO2 gas were set as 80, 4.5, 9.3, 2.1, and 1.01, respectively. Within the water, the magnitude of the electric field ranged from 1000 V/m at the bottom center to 50500 V/m at the right corner of the water surface. Accordingly, the polarization density ranged from 0.001 mC/m2 at the bottom center to 0.039 mC/m2 at the top right corner. Owing to the shape and location of the upper electrode, the electric field magnitude and the polarization density in water appeared to increase approaching the edge of the quartz substrate (Figure S1). From the perspective of the order-of-magnitude analysis, an electric field of ∼104 V/m was applied to the hydrate-forming water in this study. The molecular-scale computations by Svevkunov and Vergiri36 and Sun et al.37 showed that electric fields of ∼107−109 V/m rendered the polarization of water dipoles parallel to the direction of the electric field. Furthermore, a study using the sum frequency generation spectroscopy reported that an electric field of 107 V/m resulted in the alignment of water molecules.38 Therefore, the electric field applied in this study was expected to cause the polarization of water molecules, particularly those near the water surface,

3. RESULTS 3.1. Enhanced Gas Hydrate Nucleation Kinetics by Polarized Water Molecules. After 5 h of CO2 dissolution to water at 3 MPa, the electric field was applied. Then, the P−T condition was brought within the hydrate stability region by cooling the system to 274.15 K (Case 1 in Figure 1d). This represents conditions for hydrate formation with ordered water molecules containing a fair amount of dissolved gas. In this case, we found a significant enhancement in hydrate nucleation kinetics due to the electric fields. The kinetics of gas hydrate nucleation are defined as the time required to commence hydrate nucleation under a thermodynamically stable P−T condition.2 The induction time of the CO2 hydrate nucleation was recorded by measuring the time from the moment when the P−T condition was brought into the hydrate stable zone to the moment where an exothermic temperature spike was observed (Figure 1d).30,39 Over the course of 15 tests, a lognormal distribution of induction times was observed, confirming the stochastic characteristics of nucleation kinetics D


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Environmental Science & Technology (Figure 2a and Table S1).39 The average induction time in the absence of the electric field (Case REF) was approximately 13.3 h (±5.3). Meanwhile, in the presence of an electric field of approximately 104 V m−1, the induction time of hydrate nucleation was significantly reduced to ∼2.3 h (±0.8) (Case 1) (Figure 2b). This notable enhancement of gas hydrate nucleation kinetics can be explained by the partial breakage of the hydrogenbonded water clusters by polarization of water dipoles and by the lowered thermal agitation and random movement of polarized water molecules. The nucleation process of gas hydrate crystals is heavily affected by the activity of water molecules,1 and the nucleation of gas hydrate is known to be stochastic.39 The moderate electric field applied herein (E = ∼104 V m−1) can cause bending and partial breakage of the water molecule networks clustered by hydrogen bonding, accompanying the rearrangement of water dipoles parallel to the electric field. Previous studies using molecular dynamic simulations and the sum frequency generation spectroscopy also corroborate our results, which have reported that sufficient level of electric fields (107 to 109 V m−1) disturb hydrogen bonds,36−38 and promote the transition of supercooled water to polar cubic ice.40,41 Meanwhile, the thermal energy of molecules, in the form of thermal agitation and random vibration, acts against crystal nucleation and growth.42 It is presumed that the imposition of the electric field causes reductions in the thermal energy of the water molecules and the activation energy for nucleation, thus promoting gas hydrate nucleation kinetics. 3.2. Immediate or Stochastic Nucleation of Gas Hydrate when Applying the Electric Field. Whether or not the electric field induces the immediate nucleation of CO2 hydrate was explored by applying the electric field when the P− T condition was in the hydrate stability region (Case 2). This is a debated and controversial topic on the nucleation of ice by electric fields.43,44 We found that the commencement of gas hydrate nucleation began ∼1.2 h (±1.1) on average after the application of the electric field; thus, the average induction time of hydrate nucleation was ∼2.2 h (±1.1) by adding the interval from the moment the temperature reached 281.65 K (Figure 2c,d). This result indicates that even in the metastable state, the sudden rotational movement of water molecules created by the external electric field does not immediately initiate hydrate nucleation. Instead, the stochastic behavior prevails. Nevertheless, the nucleation kinetics were faster than the REF case, confirming the enhancement of gas hydrate nucleation kinetics by the polarized water molecules. 3.3. Retardation of Gas Dissolution and Hydrate Nucleation by Structured Water Layers. We found that the presence of strongly polarized water layers at the water−gas interface impeded gas uptake and hence the nucleation of the gas hydrate. As clay minerals are deposited in natural geologic environments, water molecules are first structured close to the clay surfaces and hydrate-forming gases then diffuse and migrate from the deeper regions to the clay-rich sediments. To investigate the nucleation kinetics of hydrate in this condition, an external electric field was applied prior to gas dissolution (Case 3 in Figure 1d). Surprisingly, in all five attempts, nucleation of CO2 hydrate was not observed within 72 h (Table S1). This was attributed to the retardation of gas uptake and lowered gas solubility due to the strongly structured water molecules at the water−vapor interface. As the electric field and the polarization density were estimated to be greatest at the

interface between water and gas vapor (see FEM calculations in Figure S1), it can be presumed that the water molecules at the water−vapor interface are the most strongly structured, impeding CO2 molecule uptake and lowering the gas solubility, leading to an insufficient amount of excess CO2 gas in water necessary to form clathrate hydrate. Our observations are consistent with previous studies that have shown that electric field application generates ice-like structures and lowers the diffusion coefficient by approximately 1 order of magnitude.45,46

4. DISCUSSION: RELEVANCE TO A DIFFUSE DOUBLE LAYER (DDL) ON A NEGATIVELY CHARGED CLAY SURFACE This study explores how the presence of polarized water molecules by electric field can trigger gas hydrate nucleation processes (Figure 3a,b). This condition is expected to occur on negatively charged clay surfaces where adsorbed water molecules are polarized. In this section, the magnitudes of electric field (or electric field strength) on natural clay surfaces are estimated, and their implications to gas hydrate nucleation in oceanic montmorillonite-rich sediments are discussed. The electric field strength on a clay surface can be calculated based on the classic double layer theory.47 Herein, let us explore the montmorillonite clay mineral with the specific surface area Ss of ∼700 m2/g and the cation exchange capacity CEC of 7.5 × 10−4 molc/g or 72.4 C/g, and thus the charge density σp (defined as CEC/Ss) of 0.103 C/m2 in 0.5 M NaCl solution at 283.15 K.48 As the surface charge density of montmorillonite clay minerals is typically suggested to be ∼0.1 C/m2,49−51 the selected values in our study are consistent with the literature. According to the Gouy−Chapman theory,52,53 the potential at the surface ψ0 is related to the surface charge density σp as follows: ⎛ zFψ0 ⎞ σp = (8R Tεrε0c)1/2 sinh⎜ ⎟ ⎝ 2RT ⎠

(1)

where εr is the dielectric constant of water (78.5 at 298 K), ε0 is the permittivity of free space (8.854 × 10−12 C2 J−1 m−1), c is the molar concentration of electrolyte in the bulk solution (mol m−3), and z is the valence of the ion. T is the absolute temperature (K), R is the gas constant (8.3145 J mol−1 K−1), and F is the Faraday constant (96485 C/mol). Then, the electric potential at a given distance can be represented by the exponential decay, as follows: ψ (x) = ψ0 exp( −κx)

(2)

where ψ0 is the potential at the surface and x is the distance from the surface in meters. κ is the inverse of the Debye length (m−1), and for a monovalent electrolyte it is defined as follows: ⎛ 2F 2c ⎞1/2 κ=⎜ ⎟ ⎝ εrε0RT ⎠

(3)

This Debye length 1/κ is typically used as the thickness of the double diffuse layer (DDL) on a clay surface. The electric field strength E(x) (V/m) at a given distance x from the clay surface at different electrolyte concentrations can be calculated with the following equation: E


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Environmental Science & Technology E (x ) =

⎤ 2RTc ⎡ ⎛ zFψ (x) ⎞ ⎟ − 1⎥ ⎢exp⎜ εrε0 ⎣ ⎝ RT ⎠ ⎦

inherently affected by the interactions between the clay minerals and water molecules from the nucleation stage to the growth stage. As, it has been proposed that CO2 be stored as hydrates in cold sediments, this option inevitably involves or even can intentionally exploit hydrate formation in clay-rich geologic formations.14,30,55 In all cases, an understanding of the association between gas hydrates and clay minerals is required as it is expected to play a significant role in the exploitation of methane production from hydrate deposits and CO2 storage in oceanic sediments. Particularly from the thermodynamic perspective of hydrate formation, small pore sizes and high ionic concentrations in clay-rich sediments negatively impact the gas hydrate phase equilibria, shifting the pressure−temperature equilibrium boundaries to harsher conditions (to lower temperature and higher pressure). Therefore, the base of methane hydrate stability zone (BMHSZ) is readily lifted by several tens of meters, thereby leading to the shallower methane hydrate stability zones (MHSZs) in clayey sediments. The actual depth to the BMHSZ, for example, was found to be 400 mbsf in the Kumano Basin, offshore Japan (Integrated Ocean Drilling Program Site C0002), however, the predicted depth to the BMHSZ was 428 mbsf.56 However, methane hydrate can still favorably form at any depth above the actual BMHSZ, and the presence of clay minerals will further promote nucleation of methane hydrate prior to the hydrate growth stage, as shown in this study. We presume that natural accumulations of gas hydrates in clay-rich sediments, such as those in the South China Sea are partially attributable to the promotion of hydrate nucleation by polarized water molecules on clay minerals.20,21 Accordingly, the observed enhanced nucleation kinetics of gas hydrates on clay minerals provides an explanation of why and how natural gas hydrates are widely found in clay-rich sediments in spite of factors that limit gas hydrate formation, such as small pore size and high salinity of clay-rich sediments. Furthermore, it provides insights into the formation of hydrates in drilling wells, which can cause severe clogging problems due to the use of drilling fluids that contain smectite-type clay minerals.57

(4)

For near-seafloor conditions with 0.5 M NaCl electrolyte at 283.15 K, the potential ψ0 and electric field E0 at the montmorillonite clay surface (where x = 0) are calculated to be approximately −52 mV and 5 × 106 V m−1, respectively. The potential and electric field values exponentially decrease as the distance from the clay surface increases (Figure 3c). The distances at which the magnitudes of electric fields become 105 V m−1, 104 V m−1, and 103 V m−1 are approximately 3, 5, and 7 nm away from the clay surface, respectively. It appears that the electric field used in this study (i.e., E = ∼104 V m−1) is equivalent to the electric field in the region 25 nm from the clay surface for 0.01 M NaCl electrolyte, and this distance decreases to 4 nm as the NaCl concentration increases to 1 M (Figure 3c). For comparison, the surface potential ψ0 and the electric field E0 at the montmorillonite clay surface in a low salinity condition with 0.1 M Ca(NO3)2 electrolyte was −78.0 mV and 2.5 × 108 V m−1, respectively.54 Accordingly, the distances with electric fields of 105 V m−1, 104 V m−1, and 103 V m−1 were suggested to be approximately 6, 8, and 11 nm, respectively, in 0.1 M Ca(NO3)2 electrolyte.54 This validates our computed values for the electric field strength. Given a DDL thickness (1/κ) of 0.3−3 nm for the montomorillonite minerals, the electric field used in this study (E = ∼104 V m−1) corresponds to the region further away outside the DDL. Thereby, the water molecules outside the DDL, subjected to the tested electric field, are presumed to be weakly polarized by the surface potential of clay minerals compared to the ones inside the DDL. Thus, for an oceanic condition with a salinity equivalent to 0.5 M NaCl, the observed test results imply that the weakly polarized water molecules, which are approximately 5 nm away from the clay surface, are more prone to hydrate formation than those in bulk water in the far field (Figure 3d). Meanwhile, the adsorbed water layers within 4 nm distance from the surface are strongly structured and too stable, such that the diffusion of hydrateforming gas is hindered and the formation of clathrate hydrate with the strongly bounded water is unfavorable. The obtained test results provide evidence supporting that an electric field of ∼104 V/m can enhance hydrate nucleation kinetics; however, further research is warranted to explore the effect of increased or decreased electric field strengths, including the minimum electric field that can affect such hydrate nucleation kinetics. Herein, we studied hydrate nucleation kinetics with DIW despite the pore water chemistry in oceanic clayey sediments shows a wide range of variations, such as pH, ionic strength, and ion types. For instance, an increase in ionic strength in pore water is expected to not only reduce the DDL thickness but also weaken the polarization of water molecules, consequently compensating the promotion effect by negatively charged clay surfaces. Accordingly, the effect of pore water chemistry on hydrate nucleation kinetics in the presence of electric fields also needs to be further investigated.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.est.7b05477. Summary table of the hydrate formation test results (Table S1) and the finite element analysis of the spatial distribution of the electric field and polarization density in water (Figure S1) (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Phone: +82-42-350-3628. Fax: +82-42-869-3610. E-mail: t. [email protected]. ORCID

Tae-Hyuk Kwon: 0000-0002-1610-8281 5. IMPLICATIONS FOR HYDRATE OCCURRENCE IN CLAY-RICH SEDIMENTS Recent gas hydrate drilling expeditions in the South China Sea (GMGS-1 and GMGS-2) have reported the unexpected abundance of gas hydrates in oceanic clay-rich sediments.20,21 These natural gas hydrate accumulations in clay formationsare

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful to the Associate Editor, John C. Crittenden, and anonymous reviewers for their thorough F


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reviews and constructive comments. This research was supported by the Korea Institute of Energy Technology Evaluation and Planning (KETEP) and the Ministry of Trade, Industry and Energy (MOTIE) of the Republic of Korea (Grant 20152520100760) and a grant (Grant 17CTAPC129729-01) from Technology Advancement Research Program (TARP) funded by Ministry of Land, Infrastructure and Transport of the Korean Government.

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